Fuel cell system

ABSTRACT

A supply channel through which an oxygen-containing exhaust gas discharged from a fuel cell stack is supplied to an exhaust gas combustor is branched so as to provide an oxygen-containing exhaust gas bypass channel through which the oxygen-containing exhaust gas is discharged to the outside in a manner to bypass the exhaust gas combustor. In the structure, the exhaust gas flow rate of an exhaust gas discharged through a condenser (saturated water vapor quantity) is suppressed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2019-122898 filed on Jul. 1, 2019, thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a fuel cell system including a fuelcell stack formed by stacking a plurality of fuel cells which performpower generation by electrochemical reactions of a fuel gas and anoxygen-containing gas.

Description of the Related Art

In general, a solid oxide fuel cell (SOFC) employs a solid electrolyte.The solid electrolyte is an oxide ion conductor such as stabilizedzirconia. The solid electrolyte is interposed between an anode and acathode to form an electrolyte electrode assembly (MEA). The electrolyteelectrode assembly is sandwiched between separators (bipolar plates). Inuse, generally, predetermined numbers of the electrolyte electrodeassemblies and the separators are stacked together to form a fuel cellstack.

As the fuel gas supplied to the fuel cell stack of SOFC, normally, ahydrogen gas produced from hydrocarbon raw material by a reformer isused. In general, in the reformer, a reforming raw gas is obtained froma hydrocarbon raw fuel of a fossil fuel or the like, such as methane orLNG, and thereafter, the reforming raw gas undergoes, e.g., steamreforming, whereby a reformed gas (fuel gas) is produced.

The steam (water vapor) to be supplied to the reformer is produced asfollows. That is, a fuel exhaust gas and an oxygen-containing exhaustgas discharged from the fuel cell stack are combusted in the exhaust gascombustor, and the resulting combustion gas passes through an evaporatorto which water is supplied. Thus, the water vapor is produced.

In this case, as disclosed in Japanese Laid-Open Patent Publication No.2013-073903 (hereinafter referred to as JPA2013-073903), the combustiongas containing water vapor is condensed in the condenser to producewater, and the produced water is collected into a water tank. In thismanner, it is possible to perform water self-sustaining operation wherethere is no need to supplement water from the outside (paragraph [0003]of JPA2013-073903).

SUMMARY OF THE INVENTION

By the way, in the SOFC disclosed in JPA2013-073903, if the temperatureof the fuel cell stack is high, in order to cool the fuel cell stack forthe purpose of avoiding damage to the fuel cell stack, the supply flowrate of the oxygen-containing gas is increased (paragraph [0012] ofJPA2013-073903).

However, if the supply flow rate of the oxygen-containing gas isincreased, the quantity of water vapor contained in the combustion gas(exhaust gas) discharged from the condenser is increased, and thequantity of water collected into the water tank is decreased.Consequently, it becomes difficult to perform water self-sustainingoperation.

Further, if the supply flow rate of the oxygen-containing gas isincreased, the combustion temperature of the combustion gas in theexhaust gas combustor is decreased. Consequently, degradation ofemission and/or accidental fire may occur.

The present invention has been made taking the above problems intoaccount, and an object of the present invention is to provide a fuelcell system which makes it possible to maintain water self-sustainingoperation, avoid damage to a fuel cell stack, and also avoid occurrenceof degradation of emission and accidental fire in an exhaust gascombustor.

A fuel cell system according to an aspect of the present inventionincludes a fuel cell stack including a plurality of fuel cells stackedtogether, the fuel cells being configured to perform power generation byelectrochemical reactions of a fuel gas and an oxygen-containing gas, areformer configured to perform steam reforming of raw fuel chieflycontaining hydrocarbon to generate the fuel gas supplied to the fuelcell stack, an exhaust gas combustor configured to generate a combustiongas by combusting a fuel exhaust gas and an oxygen-containing exhaustgas discharged from the fuel cell stack, a heat exchanger configured toperform heat exchange between the combustion gas and theoxygen-containing gas, an oxygen-containing gas supply channelconfigured to supply the oxygen-containing gas to the fuel cell stackthrough the heat exchanger, a condenser configured to condense watervapor in the combustion gas and collect water, and a control unitconfigured to control the power generation. A supply channel throughwhich the oxygen-containing exhaust gas discharged from the fuel cellstack is supplied to the exhaust gas combustor is branched so as toprovide an oxygen-containing exhaust gas bypass channel through whichthe oxygen-containing exhaust gas is discharged in a manner to bypassthe exhaust gas combustor.

In the present invention, the oxygen-containing exhaust gas bypasschannel through which the oxygen-containing exhaust gas from the fuelcell stack is discharged to the outside in a manner to bypass theexhaust gas combustor is provided. In the structure, the flow rate ofthe oxygen-containing exhaust gas to be supplied to the exhaust gascombustor is suppressed to thereby suppress the flow rate of the exhaustgas discharged from the condenser and increase the flow rate of theoxygen-containing gas supplied to the fuel cell stack. Accordingly, itis possible to maintain the water self-sustaining operation and avoiddamage to the fuel cell stack, and it is also possible to avoidoccurrence of degradation of emission and/or accidental fire in theexhaust gas combustor.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing structure of a fuel cellsystem according to an embodiment of the present invention;

FIG. 2 is a graph showing water self-sustaining operation limitcorresponding to the presence/absence of a bypass channel of anoxygen-containing exhaust gas;

FIG. 3 is a diagram schematically showing a fuel cell system forexplanation of water self-sustaining operation;

FIG. 4 is a graph of air flow rate characteristics indicating the airflow rate upper limit relative to the power generation output of thefuel cell system;

FIG. 5 is a flow chart (1/2) illustrating operation of the fuel cellsystem according to the embodiment;

FIG. 6 is a flow chart (2/2) illustrating operation of a fuel cellsystem according to the embodiment;

FIG. 7 is a view of parameters and variables used for explainingoperation; and

FIG. 8 is a diagram schematically showing structure of a fuel cellsystem according to a modified embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [Structure]

A fuel cell system 10 according to an embodiment of the presentinvention shown in FIG. 1 is used in a stationary application.Additionally, the fuel cell system 10 may be used in variousapplications. For example, the fuel cell system 10 is mounted in avehicle. A raw fuel supply apparatus (including a raw fuel pump 12) 14for supplying raw fuel (e.g., city gas) and an oxygen-containing gassupply apparatus (including an air pump 16) 18 for supplying anoxygen-containing gas are connected to the fuel cell system 10.

Further, the fuel cell system 10 includes a stack type fuel cell stack20, a reformer 22, a condenser 23, a heat exchanger 24, anevaporator/mixer 25, an exhaust gas combustor 26, and a water tank 27.

Further, the fuel cell system 10 includes an output adjustment device100 for supplying electrical energy to a load 106, an electric storagedevice 102, and a control unit 104.

The fuel cell system 10 generates electrical energy required by the load106 under control of the control unit 104, and supplies the electricalenergy to the load 106 through the output adjustment device 100.

The fuel cell stack 20 includes solid oxide fuel cells (fuel cells) 30for generating electrical energy in an electrochemical reactions of afuel gas (gas obtained by mixing a hydrogen gas with methane, and carbonmonoxide), and an oxygen-containing gas (the air). A plurality of thefuel cells 30 are stacked together.

The fuel cell 30 includes an electrolyte electrode assembly (MEA) 38including an electrolyte 32, and a cathode 34 and an anode 36 providedon both sides of the electrolyte 32. The electrolyte 32 is made of oxideion conductor such as stabilized zirconia.

A cathode side separator 40 and an anode side separator 42 are providedon both sides of the electrolyte electrode assembly 38. Anoxygen-containing gas flow field 44 for supplying the oxygen-containinggas to the cathode 34 is formed on the cathode side separator 40, and afuel gas flow field 46 for supplying the fuel gas to the anode 36 isformed on the anode side separator 42. It should be noted that, it ispossible to use, as the fuel cell 30, any of various types ofconventional SOFCs.

The fuel cell 30 is operated at an operating temperature of severalhundred degrees (° C.), and a fuel gas (hydrogen) which has beenreformed at the reformer 22 is supplied to the anode 36.

An oxygen-containing gas supply passage 48 a and an oxygen-containinggas discharge passage 48 b are provided in the fuel cell stack 20. Theoxygen-containing gas supply passage 48 a is connected to an inlet sideof each oxygen-containing gas flow field 44, and the oxygen-containinggas discharge passage 48 b is connected to an outlet side of eachoxygen-containing gas flow field 44. The oxygen-containing gas supplypassage 48 a and the oxygen-containing gas discharge passage 48 b extendthrough the fuel cell stack 20 in the stacking direction.

A fuel gas supply passage 50 a and a fuel gas discharge passage 50 b areprovided in the fuel cell stack 20. The fuel gas supply passage 50 a isconnected to an inlet side of each fuel gas flow field 46, and the fuelgas discharge passage 50 b is connected to an outlet side of the fuelgas flow field 46. The fuel gas supply passage 50 a and the fuel gasdischarge passage 50 b extend through the fuel cell stack 20 in thestacking direction.

The evaporator/mixer 25 is made up of an evaporator and a mixer. Theevaporator turns the water into water vapor. The mixer mixes raw fuelchiefly containing hydrocarbon with water vapor, and supplies theresulting gas as a fuel gas to the reformer 22.

That is, the evaporator/mixer 25 evaporates the water supplied from thewater tank 27 to produce water vapor by the heat absorbed from thecombustion gas supplied from the exhaust gas combustor 26.

The reformer 22 includes a reforming catalyst. The reformer 22 reforms afuel gas mixed with water vapor to produce the fuel gas supplied to thefuel cell stack 20.

The heat exchanger 24 heats the oxygen-containing gas by heat exchangewith the combustion gas supplied through the evaporator in theevaporator/mixer 25, and supplies the heated oxygen-containing gas tothe fuel cell stack 20.

The exhaust gas combustor 26 combusts the fuel exhaust gas which is thefuel gas discharged from the fuel cell stack 20 and theoxygen-containing exhaust gas which is the oxygen-containing gasdischarged from the fuel cell stack 20 to produce a hot combustion gas,and supplies the combustion gas to the evaporator in theevaporator/mixer 25 through the reformer 22.

The condenser 23 liquefies the overheated water vapor contained in thecombustion gas supplied through the evaporator/mixer 25 and the heatexchanger 24, collects the water into the water tank 27, and dischargesthe heat as the exhaust gas to the outside through an exhaust gaschannel 67, a flow rate adjustment unit 29, and an exhaust gas channel69.

The raw fuel supply apparatus 14 includes a raw fuel supply channel 52for supplying the raw fuel to the mixer in the evaporator/mixer 25.

The oxygen-containing gas supply apparatus 18 includes anoxygen-containing gas supply channel 54 for supplying theoxygen-containing gas to the heat exchanger 24, and an oxygen-containinggas supply channel 55 for supplying the oxygen-containing gas which hasbeen subjected to heat exchange at the heat exchanger 24 to theoxygen-containing gas supply passage 48 a of the fuel cell stack 20.

One end of the oxygen-containing exhaust gas channel (exhaust gasoutlet) 60 is connected to the oxygen-containing gas discharge passage48 b of the fuel cell stack 20, and the other end of theoxygen-containing exhaust gas channel 60 is branched into anoxygen-containing exhaust gas channel 60 a and an oxygen-containingexhaust gas channel (oxygen-containing exhaust gas bypass channel) 60 b.The oxygen-containing exhaust gas channel 60 a is connected to theexhaust gas combustor 26, and the oxygen-containing exhaust gas channel(oxygen-containing exhaust gas bypass channel) 60 b is connected to theexhaust gas channel 69.

One end of a combustion gas channel 64 is connected to an outlet side ofthe exhaust gas combustor 26, and the other end of the combustion gaschannel 64 is connected to the evaporator/mixer 25 through the reformer22.

One end of a combustion gas channel 65 is connected to an outlet side ofthe evaporator/mixer 25, and the other end of the combustion gas channel65 is connected to an inlet side of the heat exchanger 24.

The exhaust gas channel 66 for discharging the combustion gas (exhaustgas) consumed in heat exchange with the oxygen-containing gas isconnected to an outlet side of the heat exchanger 24. The condenser 23is disposed in the middle of the exhaust gas channel 66. One end of theexhaust gas channel 67 is connected to an exhaust side of the condenser23, and the other end of the exhaust gas channel 67 is connected to aninlet side of the flow rate adjustment unit 29. An outlet side of theflow rate adjustment unit 29 and an outlet side of the oxygen-containingexhaust gas bypass channel 60 b are merged together, and theoxygen-containing exhaust gas and the exhaust gas (exhaust gas of thecombustion gas) are released to the outside through the exhaust gaschannel 69.

A water supply channel 68 is connected to an inlet side of theevaporator/mixer 25, and the reformer 22 is connected to an outlet sideof the evaporator/mixer 25.

A fuel gas supply channel 58 for supplying the fuel gas to the fuel gassupply passage 50 a of the fuel cell stack 20 is connected to thereformer 22.

The fuel cell system 10 generates electrical energy required for theload 106 (hereinafter referred to as the “required power generationoutput Ld”), and supplies, as the power generation output L, the powergeneration current (hereinafter referred to as the output current) I,from one end of the stacked fuel cells 30 (i.e., the end plate (notshown) at one end in the stacking direction connected to the cathode 34at one end of the fuel cell stack 20) through an electric circuit 108,the output adjustment device 100, and an electric circuit 110, to oneend (active side) of the load 106. The other end (cold side, groundside) of the load 106 is connected to the anode 36 through an electriccircuit 111 and via an end plate (not shown) at the other end in thestacking direction connected to the anode 36. It should be noted thatthe electric paths outside the fuel cell stack 20 such as the electriccircuit 108 are denoted by thick bold lines.

The output adjustment device 100 supplies excessive power generationelectrical energy of the power generation output L (power generationcurrent I×power generation voltage) to one end (active side) of theelectric storage device 102 through an electric circuit 112. The otherend (cold side, ground side) of the electric storage device 102 isconnected to the anode 36 through an electric circuit (not shown) andthe end plate at the other end.

For example, the control unit 104 is made up of an ECU (electric controlunit) having a CPU, a storage device (storage unit), and variousinput/output interfaces. Based on input (electrical signal) from each ofcomponent parts, the control unit 104 executes a program stored in partof the storage device in the ECU, and outputs a control signal (electricsignal) to each of the component parts.

Examples of the input from each of the component parts include requiredpower generation output Ld of the load 106 set at an output setting unit(not shown) of the fuel cell system 10, water tank storage quantity(also referred to as the “water quantity”, the “detected waterquantity”) Sw detected by the water quantity sensor 114 provided at thewater tank 27, the water flow rate Qw supplied from a pump (not shown)provided at the water tank 27 to the evaporator/mixer 25 through thewater supply channel 68, the stack temperature (hereinafter alsoreferred to as the “temperature”, the “detected temperature”) Tsdetected by a temperature sensor 116 disposed adjacent to theoxygen-containing gas discharge passage 48 b in the fuel cell stack 20,power generation current I detected by a current sensor (not shown)disposed in the electric circuit 108, power generation voltage detectedby a voltages sensor (not shown) disposed in the electric circuit 108,an electrical signal corresponding to the flow rate of air dischargedfrom the air pump 16 (in the embodiment, the air flow rate Qa of the airflowing through the oxygen-containing gas supply channel 54), anelectrical signal indicating the flow rate (the raw fuel flow rate, thefuel flow rate) Qf of the raw fuel discharged from the raw fuel pump 12,an electrical signal indicating the SOC (state of charge) of theelectric storage device 102, etc.

Examples of control signals to each of the component parts include asignal for regulating the output adjustment device 100, command signalsto the oxygen-containing gas supply apparatus 18 and the flow rateadjustment unit 29, for regulating the air flow rate Qa of air suppliedfrom the air pump 16 to the oxygen-containing gas supply channel 54, anda command signal to the raw fuel supply apparatus 14 for regulating theraw fuel rate Qf of the raw fuel supplied from the raw fuel pump 12 tothe raw fuel supply channel 52.

[Operation]

With regard to the operation of the fuel cell system 10 having the abovestructure, firstly, [General Power Generation Operation] will bedescribed, and then, [Advantages Offered by Providing theOxygen-Containing Exhaust Gas Bypass Channel 60 b] will be described,and lastly, [Water Self-Sustaining Operation/Stack Temperature ControlOperation] will be described. The operation is performed through thecontrol unit 104. However, since it would be complicated to explain theoperation with reference to the control unit 104 each time, reference tothe control unit 104 will be omitted as necessary.

[General Power Generation Operation]

At the time of continuing power generation, the valve opening degree ofthe flow rate adjustment unit 29 is set in correspondence with the powergeneration output L. Under driving operation of the air pump 16, the airis supplied from the oxygen-containing gas supply apparatus 18 along theoxygen-containing gas supply channel 54 through the heat exchanger 24,to the oxygen-containing gas supply channel 55.

In the meanwhile, in the raw fuel supply apparatus 14, a raw fuel suchas the city gas (CH₄, C₂H₆, C₃H₈, C₄H₁₀), for example, is supplied tothe raw fuel supply channel 52 in correspondence with the powergeneration output L, under driving operation of the raw fuel pump 12.

The raw fuel is supplied into the evaporator/mixer 25. Further, water issupplied from the water tank 27 to the evaporator/mixer 25, and the hotcombustion gas is supplied to the evaporator/mixer 25 through thereformer 22.

The evaporator/mixer 25 turns the water supplied from the water tank 27into water vapor by the heat of the combustion gas, mixes the raw fuelwith the water vapor, and then supplies the mixed gas as a fuel gas tothe reformer 22.

The reformer 22 heats the fuel gas mixed with the water vapor by thecombustion gas for inducing reforming reaction to produce hot reductiongas (fuel gas).

The hot reduction gas (fuel gas) is supplied to the fuel gas supplychannel 58.

The hot air supplied from the air pump 16 passes through theoxygen-containing gas supply channel 54, the heat exchanger 24, and theoxygen-containing gas supply channel 55, flows through theoxygen-containing gas supply passage 48 a, and then flows through theoxygen-containing gas flow field 44 of each fuel cell 30. The hotreduction gas (fuel gas) supplied to the fuel gas supply channel 58flows through the fuel gas supply passage 50 a, and then flows throughthe fuel gas flow field 46 of each fuel cell 30.

As a result, air is supplied to the cathode 34 of each fuel cell 30, andthe fuel gas is supplied to the anode 36 of each fuel cell 30 to performpower generation by electrochemical reactions. Thus, the powergeneration electrical current I is supplied to the load 106 or theelectric storage device 102 through the output adjustment device 100.

In the fuel cell stack 20, the hot reduction gas which has flowedthrough each fuel gas flow field 46 is discharged as the fuel exhaustgas from the fuel gas discharge passage 50 b to a fuel exhaust gaschannel 62, and is then introduced through the fuel exhaust gas channel62 into the exhaust gas combustor 26.

Further, the hot air which has flowed through each oxygen-containing gasflow field 44 is discharged as the oxygen-containing exhaust gas fromthe oxygen-containing gas discharge passage 48 b into theoxygen-containing exhaust gas channel 60. The oxygen-containing exhaustgas is introduced through the oxygen-containing exhaust gas channel 60 ainto the exhaust gas combustor 26, and some of the oxygen-containingexhaust gas is discharged as an exhaust gas through theoxygen-containing exhaust gas bypass channel 60 b.

In the exhaust gas combustor 26, the air (oxygen-containing exhaust gas)and the reduction gas (fuel exhaust gas) are self-ignited, or ignited byignition means (not shown), and combusted. The hot combustion gascontaining overheated water vapor produced in the exhaust gas combustor26 flows through the combustion gas channel 64, the reformer 22, theevaporator/mixer 25, the combustion gas channel 65, and the heatexchanger 24. Then, the hot combustion gas is supplied through theexhaust gas channel 66 to the condenser 23.

In the heat exchanger 24, the air supplied from the air pump 16 throughthe oxygen-containing gas supply channel 54 is heated by the combustiongas introduced into the heat exchanger 24. The combustion gas which hasflowed through the heat exchanger 24 flows through the exhaust gaschannel 66 into the condenser 23.

In the condenser 23, some of the water vapor contained in the combustiongas is cooled, liquefied, and then the liquefied water is dischargedinto the water tank 27. The combustion gas containing the remainingwater vapor is discharged as the exhaust gas through the exhaust gaschannel 67, the flow rate adjustment unit 29, and the exhaust gaschannel 69.

Power generation operation of the fuel cell system 10 is continued inthe manner as described above.

[Advantages Offered by Providing the Oxygen-Containing Exhaust GasBypass Channel 60 b]

Next, for facilitating understanding of the present invention, theadvantages/significance of the fuel cell system 10 offered by providingthe oxygen-containing exhaust gas bypass channel 60 b in theoxygen-containing exhaust gas channel 60 will be described.

As shown in FIG. 2, in general, as the fuel cell stack 20 becomesdeteriorated, heat generation becomes increased. As such, the air flowrate Qa is increased to maintain the stack temperature Ts at an upperlimit or less. However, the air flow rate may exceed the air flow rateupper limit (water self-sustaining operation limit) (referred to as the“without bypass” in the case where the oxygen-containing exhaust gasbypass channel 60 b is not provided).

That is, in order to reduce damage to the fuel cell stack 20 by keepingthe stack temperature Ts at a temperature not more than the upper limit,it is desirable to flow the air through the fuel cell 30 at a larger airflow rate Qa. However, if the air flow rate Qa is increased, the flowrate of combustion gas is increased and the quantity of saturated watervapor discharged as the exhaust gas from the condenser 23 is increased.As a result, the quantity of water condensed in the condenser 23 isdecreased, and thus it is not allowed to increase the air flow rate Qato the air flow rate upper limit (water self-sustaining operation limit)(without bypass) or more.

In order to deal with the above situation, in the fuel cell system 10according to the embodiment of the present invention, as shown in adiagram of FIG. 3 (diagram for illustrating water self-sustainingoperation) where water circulation is focused, the oxygen-containingexhaust gas bypass channel 60 b (also see FIG. 1) configured to connectthe oxygen-containing exhaust gas channel 60 of the fuel cell stack 20to the exhaust gas channel 69 extending from the condenser 23 isprovided, and the flow rate adjustment unit 29 such as the flow rateregulator valve is provided.

As described above, some of the oxygen-containing exhaust gas beforecombustion (dry hot gas) is caused to bypass the exhaust gas combustor26 by using the oxygen-containing exhaust gas bypass channel 60 b, andthe exhaust gas flow rate of the exhaust gas flowing through the flowrate adjustment unit 29 is regulated so as to become low, for example,by the flow rate adjustment unit 29. In this manner, the bypass exhaustgas flow rate of the bypass exhaust gas passing through theoxygen-containing exhaust gas bypass channel 60 b is increased, and theflow rate of the combustion gas (containing saturated water vapor) whichpasses through the condenser 23 and is then discharged from the exhaustgas channel 69 is decreased.

By providing the oxygen-containing exhaust gas bypass channel 60 b asdescribed above, as shown by a blank upward arrow in FIG. 2, the waterself-sustaining operation limit for the air flow rate Qa is eased from(water self-sustaining operation limit) (without bypass) to (waterself-sustaining operation limit) (with bypass), to thereby make itpossible to collect a sufficient amount of water at the condenser 23.

Therefore, in the case where the fuel cell stack 20 is deteriorated overtime and heat generation is increased, the air flow rate Qa of the airsupplied to the fuel cell stack 20 is increased to thereby suppressraise in the temperature of the fuel cell stack 20, and at the sametime, the increased amount of air is discharged through theoxygen-containing exhaust gas bypass channel 60 b. In this manner, it ispossible to ensure that water self-sustaining operation is performed.

Since it is possible to increase the air flow rate of the air suppliedto the fuel cell stack 20 from the initial stage of operation of thefuel cell stack 20, it is possible to reduce (equalize) the stacktemperature distribution, and extend the product life of the fuel cellstack 20. Further, since the flow rate of the oxygen-containing exhaustgas supplied to the exhaust gas combustor 26 is reduced, the combustiontemperature is increased, and thus it is possible to suppress occurrenceof degradation of emission and accidental fire.

FIG. 4 is a graph showing the air flow rate characteristics (air flowrate map) 120 according to the embodiment, for determining the air flowrate Qa required for obtaining the power generation output L, the mapbeing stored in a storage device (storage unit) of the control unit 104beforehand.

The air flow rate characteristics 120 have good cost performance of thebalance of plant (BOP: peripheral devices other than the fuel cell stack20), which can be set in the fuel cell system 10 having theoxygen-containing exhaust gas bypass channel 60 b.

The air flow rate Qa in the vertical axis represents the flow rate atwhich the air is supplied from the air pump 16, through theoxygen-containing gas supply channels 54, 55 and the oxygen-containinggas supply passage 48 a, to the oxygen-containing gas flow field 44formed in the cathode 34 of the fuel cell stack 20. It should be notedthat the air flow rate Qa is controlled by the air pump 16.

Since the power generation output L in the horizontal axis isproportional to the air flow rate Qa, the air flow rate lower limit Qlcorresponding to the upper limit (oxygen utilization ratio limit) of theset oxygen utilization ratio (ratio of the oxygen quantity of the oxygenconsumed in power generation of the fuel cell stack 20 relative to theoxygen quantity of oxygen supplied to the fuel cell stack 20) Ro isdetermined.

When the fuel cell stack 20 is deteriorated, heat generation becomeslarge, and it is required to increase the air flow rate Qa for the samepower generation output L.

However, when the air flow rate Qa is increased, the exhaust gas flowrate of the exhaust gas discharged from the condenser 23 is increased,and the water quantity discharged as the saturated water vapor isincreased. Therefore, it becomes impossible to perform waterself-sustaining operation for collecting and recirculating the waterliquefied in the condenser 23.

In order to ensure that the water self-sustaining operation isperformed, in the fuel cell system 10, the air flow rate upper limit Qhcorresponding to the water self-sustaining operation limit (withbypass), in particular, in consideration of the product life of the fuelcell stack 20 is set.

Therefore, in the air flow rate characteristics 120 where BOP of theheat balance that can be adopted in the fuel cell system 10, accordingto the embodiment, having the oxygen-containing exhaust gas bypasschannel 60 b provided in parallel to the oxygen containing exhaust gaschannel 60 a is adopted, on the high power generation output L (highload) side, the flow rate in the flow rate adjustment unit 29 is reducedto thereby reduce the oxygen-containing exhaust gas flow rate Qc of theoxygen-containing exhaust gas flowing from the oxygen-containing exhaustgas channel 60 a to the exhaust gas combustor 26, while theoxygen-containing exhaust gas flow rate (oxygen-containing exhaust gasbypass flow rate) Qb of the oxygen-containing exhaust gas flowing intothe oxygen-containing exhaust gas bypass channel 60 b is increasedrelatively. In this manner, the air flow rate Qa is maintained at apredetermined flow rate in correspondence with the power generationoutput L. In this case, since it is possible to reduce the quantity ofthe water vapor which is carried off by the exhaust gas of thecombustion gas passing through the exhaust gas combustor 26, thereformer 22, the evaporator/mixer 25, and the heat exchanger 24 andfurther through the condenser 23, it is possible to collect a greateramount of water by the condenser 23 owing to the reduced water vaporquantity. Thus, it is possible not to exceed the air flow rate upperlimit (water self-sustaining operation limit) Qh.

[Water Self-Sustaining Operation/Stack Temperature Control Operation]

Next, [Water Self-Sustaining Operation/Stack Temperature ControlOperation] will be described. In this case, this operation will bedescribed with reference to flow charts 1/2, 2/2 shown in FIGS. 5 and 6corresponding to a program executed by the control unit 104. Since itwould be complicated to explain the operation with reference to thecontrol unit 104 each time, reference to the control unit 104 will beomitted as necessary.

FIG. 7 shows parameters and variables used for describing operation ofeach of the flow charts.

In the process according to the flow charts, in the entire preset rangeof the power generation output L (the lateral axis of the hatched areacorresponding to the air flow rate characteristics 120 in FIG. 4), theair flow rate Qa is controlled to fall within the range of the hatchedarea in FIG. 4 between the air flow rate lower limit (oxygen utilizationratio limit) Ql and the air flow rate upper limit (water self-sustainingoperation limit) Qh.

In step S1, the control unit 104 obtains the power generation outputrequest to the load 106 based on control operation of an input device(not shown) (hereinafter referred to as the required power generationoutput Ld).

In step S2, the control unit 104 determines the power generation currentI in correspondence with the required power generation output Ld, anddetermines the air flow rate Qa by referring to the air flow ratecharacteristics 120 (FIG. 4).

In this case, as described above, on the side where the power generationoutput of the fuel cell stack 20 is relatively high, in order tomaintain the set air flow rate Qa and collect the water in the watertank 27, the oxygen-containing exhaust gas bypass flow rate Qb which isthe flow rate of the oxygen-containing exhaust gas flowing through theoxygen-containing exhaust gas bypass channel 60 b is increased, and theoxygen-containing exhaust gas flow rate Qc at which theoxygen-containing exhaust gas is supplied to the exhaust gas combustor26 is decreased.

Further, optimum ratios of the fuel flow rate Qf and the water flow rateQw relative to the power generation current I corresponding to the setair flow rate Qa are determined (set).

In step S3, the control unit 104 determines whether or not the powergeneration output L measured by the output adjustment device 100 isequal to the required power generation output Ld.

In the case where L≤Ld, i.e., the power generation output L is not equalto the required power generation output Ld (step S3: NO), in step S4, itis determined whether or not the power generation output L is not morethan the required power generation output Ld (i.e., whether or notL≤Ld).

In the case where L≤Ld, i.e., the power generation output L is not morethan the required power generation output Ld (step S4: YES), in step S5,the air flow rate Qa, the fuel flow rate Qf, and the water flow rate Qware increased for increasing the power generation current I. It shouldbe noted that the ratios of the flow rates Qa, Qb (or Qc), Qf, Qwrelative to the power generation current I are maintained at the currentratios (in the first process, the ratios determined in step S2; in thesecond and the subsequent processes, the ratios when the routine returnsto the process through a connector 2 in the flow chart).

In the meanwhile, in the determination of step S4, in the case whereL>Ld, i.e., the power generation output L is larger than the requiredpower generation output Ld (step S4: NO), in step S6, the air flow rateQa, the fuel flow rate Qf, and the water flow rate Qw are decreased forreducing the power generation current I. It should be noted that, inthis case also, the above described current ratios of the flow rates Qa,Qb (or Qc), Qf, Qw relative to the power generation current I aremaintained.

In the above described step S3, in the case where L=Ld, i.e., the powergeneration output L is equal to the required power generation output Ld(step S3: YES), or in the case where the processes of steps S5 and S6are finished, in step S7 (FIG. 6), it is determined whether or not thewater tank storage quantity S is not more than the water self-sustainingoperation alarming water quantity Sl (i.e., whether S≤Sl).

In the case where the water tank storage quantity S is less than thewater self-sustaining operation alarming water quantity Sl (step S7:YES), i.e., in the case where, if control (power generation) iscontinued under the same condition, shortage or depletion of the watertank storage quantity S may occur, in step S8, the flow rate adjustmentunit 29 reduces the flow rate of the exhaust gas passing through theflow rate adjustment unit 29 by a predetermined rate, to therebyincrease the oxygen-containing exhaust gas bypass flow rate Qbaccordingly, and the routine proceeds to step S9.

In the process of step S8, the quantity of water vapor which is carriedoff as the exhaust gas by the combustion gas passing the condenser 23 isdecreased, and the amount of water commensurate with the reduced amountof the water vapor can be collected additionally into the water tank 27by the condenser 23.

In the case where the condition of determination in step S7 is notsatisfied (step S7: NO), i.e., in the case where the water tank storagequantity S in the water tank 27 is sufficient (S>Sl), the routineproceeds to step S9 without passing through step S8.

Then, in step S9, it is determined whether or not the stack temperatureTs falls within the temperature range between the stack temperaturelower limit Tl and the stack temperature upper limit Th (i.e., whetherTl≤Ts≤Th).

In step S9, in the case where Tl≤Ts≤Th, i.e., the stack temperature Tsfalls within the temperature range between the stack temperature lowerlimit Tl and the stack temperature upper limit Th (step S9: YES), thecurrent ratios of the air flow rate Qa and the oxygen-containing exhaustgas bypass flow rate Qb are maintained without any changes, and theroutine proceeds to step S3.

In step S9, in the case where the temperature falls out of the range(step S9: NO), in step S10, it is determined whether or not the stacktemperature Ts is less than the stack temperature lower limit Tl.

In the case where Ts<Tl, i.e., the stack temperature Ts is below thestack temperature lower limit Tl (step S10: YES), then, in step S11, itis determined whether or not the air flow rate Qa is the air flow ratelower limit (oxygen utilization ratio limit) Ql.

In the case where Qa=Ql (step S11: YES), in order to increase the stacktemperature Ts, since it is not possible to decrease the air flow rateQa anymore in this case, in step S12, the flow rate of the raw fueldischarged from the raw fuel pump 12 is increased by a predeterminedquantity to thereby increase the fuel gas flow rate Qf by apredetermined rate. By increasing the flow rate of the fuel gas (fuelflow rate) Qf and utilizing the combustion gas having the increasedcombustion temperature in the exhaust gas combustor 26, it is possibleto increase the temperature of the fuel gas through the evaporator/mixer25 and the reformer 22, and also increase the temperature of theoxygen-containing gas through the heat exchanger 24. As a result, it ispossible to increase the stack temperature Ts of the fuel cell stack 20.In this case, the current ratios of the fuel flow rate Qf and the waterflow rate Qw are maintained, and the routine proceeds to step S3.

In the case where the condition of determination in step S11 is notsatisfied (Qa>Ql), in step S13, the air flow rate Qa is decreased by apredetermined rate. In this manner, by utilizing the combustion gashaving the increased combustion temperature in the exhaust gas combustor26, it is possible to increase the temperature of the fuel gas throughthe evaporator/mixer 25 and the reformer 22, and the temperature of theoxygen-containing gas through the heat exchanger 24, and also increasethe raise in the air temperature by the heat generation in the fuel cellstack 20. Therefore, it is possible to increase the stack temperatureTs. In this case, the current ratios of the air flow rate Qa and theoxygen-containing exhaust gas bypass flow rate Qb are maintained withoutany changes, and the routine proceeds to step S3.

In the case where the condition of determination in the above describedstep S10 is not satisfied (Ts>Th), in step S14, while the current ratiosof the air flow rate Qa and the oxygen-containing exhaust gas bypassflow rate Qb are maintained, the air flow rate Qa is increased tothereby decrease the stack temperature Ts of the fuel cell stack 20, andthe routine proceeds to step S3.

In this case, the air flow rate Qa is increased by a predetermined rateso that the temperature of the fuel gas can be decreased by thecombustion gas having the decreased combustion temperature in theexhaust gas combustor 26 through the evaporator/mixer 25 and thereformer 22, and the temperature of the oxygen-containing gas throughthe heat exchanger 24, and the raise in the air temperature by the heatgeneration in the fuel cell stack 20 can be decreased. Therefore, it ispossible to decrease the stack temperature Ts.

Modified Embodiment

The following modification may be made. The constituent elements havingthe same structure as those of the above described embodiment arelabeled with the same reference numerals, and only the differences willbe described.

FIG. 8 is a diagram showing structure of a fuel cell system 10Aaccording to a modified embodiment. In the fuel cell system 10Aaccording to this modified embodiment, the heat exchanger 24 is disposedbetween the exhaust gas combustor 26 and the reformer 22. In this fuelcell system 10A, since the temperature of the oxygen-containing gas canbe increased, it is possible to suitably use the fuel cell system 10A incold regions.

Invention which can be Understood from the Embodiment and the ModifiedEmbodiment

Hereinafter, the invention which can be understood from the aboveembodiment and the modified embodiment will be described. For ease ofunderstanding, the constituent elements are labeled with the referencenumerals that are used in the above embodiment and the modifiedembodiment. However, the constituent elements are not limited to thoselabeled with the reference numerals.

The fuel cell system 10, 10A according to the present inventionincludes:

the fuel cell stack 20 including the plurality of fuel cells 30 stackedtogether, the fuel cells being configured to perform power generation byelectrochemical reactions of the fuel gas and the oxygen-containing gas;

the reformer 22 configured to perform steam reforming of raw fuelchiefly containing hydrocarbon to generate the fuel gas supplied to thefuel cell stack 20;

the exhaust gas combustor 26 configured to generate a combustion gas bycombusting the fuel exhaust gas and the oxygen-containing exhaust gasdischarged from the fuel cell stack 20;

the heat exchanger 24 configured to perform heat exchange between thecombustion gas and the oxygen-containing gas;

the oxygen-containing gas supply channel 55 configured to supply theoxygen-containing gas to the fuel cell stack 20 through the heatexchanger 24;

the condenser 23 configured to condense water vapor in the combustiongas and collect water; and

the control unit 104 configured to control the power generation, and

the supply channel through which the oxygen-containing exhaust gasdischarged from the fuel cell stack 20 is supplied to the exhaust gascombustor 26 is branched so as to provide the oxygen-containing exhaustgas bypass channel 60 b through which the oxygen-containing exhaust gasis discharged in a manner to bypass the exhaust gas combustor 26.

As described above, in the case where the stack heat generation quantityis increased due to deterioration of the fuel cell stack 20 over time,the oxygen-containing gas flow rate (air flow rate Qa) of theoxygen-containing gas supplied to the fuel cell stack 20 is increased tocool the fuel cell stack 20, and the oxygen-containing exhaust gas fromthe fuel cell stack 20 is caused to flow through the oxygen-containingexhaust gas bypass channel 60 b for bypassing the exhaust gas combustorto thereby suppress the flow rate of the oxygen-containing exhaust gassupplied to the exhaust gas combustor 26, so that the flow rate of theexhaust gas discharged from the condenser 23 can be suppressed.

Since the exhaust gas flow rate of the exhaust gas which is dischargedthrough the condenser 23 (saturated water vapor amount which is carriedoff) is suppressed, it becomes possible to suitably collect water at thecondenser 23, and perform water self-sustaining operation.

Therefore, even if the fuel cell stack 20 is deteriorated over time, itis possible to avoid damage to the fuel cell stack 20, and perform waterself-sustaining operation (see FIG. 2).

It should be noted that since it is possible to increase the air flowrate Qa from the initial stage of operation of the fuel cell stack 20,it is possible to reduce (equalize) the stack temperature distribution,and extend the product life of the fuel cell stack 20.

In this case, since the oxygen-containing exhaust gas flow rate of theoxygen-containing exhaust gas supplied to the exhaust gas combustor 26is small, the combustion temperature in the exhaust gas combustor 26 isincreased, and it is possible to reduce occurrence of degradation ofemission and accidental fire.

The fuel cell system further includes the flow rate adjustment unit 29configured to regulate the flow rate of the oxygen-containing exhaustgas supplied to the exhaust gas combustor 26. The control unit 104 isconfigured to increase the flow rate of the oxygen-containing exhaustgas branching off into the oxygen-containing exhaust gas bypass channel60 b by using the flow rate adjustment unit 29 to thereby increase thequantity of water collected at the condenser 23, when it is predictedthat the quantity of water collected at the condenser 23 is decreased tobe so small that water self-sustaining operation cannot be maintained.

In the structure, since the flow rate of the combustion gas containingthe saturated water vapor supplied to the condenser 23 is decreased, thewater vapor quantity of the water vapor carried off from the condenser23 to the outside through the exhaust gas channel 67, the flow rateadjustment unit 29, and the exhaust gas channel 69 is decreased, and itis possible to increase the quantity of water collected at the condenser23.

The fuel cell system further includes the storage unit configured tostore characteristics defining the oxygen-containing gas flow rate lowerlimit Ql as the lower limit of the flow rate of the oxygen-containinggas relative to the power generation output L of the fuel cell stack 20;and

the temperature sensor 116 configured to detect the stack temperature Tsof the fuel cell stack 20, and

the control unit 104 is configured to increase the flow rate Qf of thefuel gas, when the stack temperature Ts is decreased below the thresholdtemperature Tl and the flow rate Qa of the oxygen-containing gas is atthe oxygen-containing gas flow rate lower limit Ql (step S11: YES→stepS12).

As described above, the flow rate of the fuel gas (fuel flow rate) Qf isincreased to increase the combustion temperature of the combustion gasin the exhaust gas combustor 26. Utilizing the combustion gas, it ispossible to increase the temperature of the fuel gas through theevaporator/mixer 25 and the reformer 22, and the temperature of theoxygen-containing gas through the heat exchanger 24. As a result, it ispossible to increase the stack temperature Ts of the fuel cell stack 20.

Preferably, the fuel cell system further includes the output adjustmentdevice 100 configured to be controlled by the control unit 104 andadjust the output current I to the load 106, and the control unit 104 isconfigured to increase the output current I when power generation outputof the fuel cell stack 20 is smaller than the required power generationoutput Ld, and decrease the output current I when the power generationoutput L is larger than the required power generation output Ld.

In the structure, even if the power generation output L is decreased dueto deterioration of the fuel cell stack 20, and even if the heatgeneration quantity relative to the power generation output L isincreased, it is possible to regulate the output current I, and achievethe desired required power generation output Ld.

It is matter of course that the present invention is not limited to theabove described embodiments, and can adopt various structures based onthe description of this specification. For example, the flow rateadjustment unit 29 may be dispensed with, or the oxygen-containingexhaust gas bypass channel 60 b may not be connected to (merged with)the exhaust gas channel 69, whereby the oxygen-containing exhaust gas isdirectly discharged.

What is claimed is:
 1. A fuel cell system comprising: a fuel cell stackincluding a plurality of fuel cells stacked together, the fuel cellsbeing configured to perform power generation by electrochemicalreactions of a fuel gas and an oxygen-containing gas; a reformerconfigured to perform steam reforming of raw fuel chiefly containinghydrocarbon to generate the fuel gas supplied to the fuel cell stack; anexhaust gas combustor configured to generate a combustion gas bycombusting a fuel exhaust gas and an oxygen-containing exhaust gasdischarged from the fuel cell stack; a heat exchanger configured toperform heat exchange between the combustion gas and theoxygen-containing gas; an oxygen-containing gas supply channelconfigured to supply the oxygen-containing gas to the fuel cell stackthrough the heat exchanger; a condenser configured to condense watervapor in the combustion gas and collect water; and a control unitconfigured to control the power generation, wherein a supply channelthrough which the oxygen-containing exhaust gas discharged from the fuelcell stack is supplied to the exhaust gas combustor is branched so as toprovide an oxygen-containing exhaust gas bypass channel through whichthe oxygen-containing exhaust gas is discharged in a manner to bypassthe exhaust gas combustor.
 2. The fuel cell system according to claim 1,further comprising a flow rate adjustment unit configured to regulate aflow rate of an oxygen-containing exhaust gas supplied to the exhaustgas combustor, wherein the control unit is configured to increase theflow rate of the oxygen-containing exhaust gas branching off into theoxygen-containing exhaust gas bypass channel by using the flow rateadjustment unit to thereby increase a quantity of water collected at thecondenser, when it is predicted that the quantity of water collected atthe condenser is decreased to be so small that water self-sustainingoperation cannot be maintained.
 3. The fuel cell system according toclaim 1, further comprising: a storage unit configured to storecharacteristics defining an oxygen-containing gas flow rate lower limitas a lower limit of a flow rate of the oxygen-containing gas relative topower generation output of the fuel cell stack; and a temperature sensorconfigured to detect a stack temperature of the fuel cell stack, whereinthe control unit is configured to increase a flow rate of the fuel gas,when the stack temperature is decreased below a threshold temperatureand the flow rate of the oxygen-containing gas is at theoxygen-containing gas flow rate lower limit.
 4. The fuel cell systemaccording to claim 1, further comprising an output adjustment deviceconfigured to be controlled by the control unit and adjust an outputcurrent to a load, wherein the control unit is configured to increasethe output current when power generation output of the fuel cell stackis smaller than required power generation output; and decrease theoutput current when the power generation output is larger than therequired power generation output.
 5. The fuel cell system according toclaim 1, wherein each of the fuel cells is a solid oxide fuel cell.